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Handle a hypothetical relative humidity of 0.0 %

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Summary:
The formular to calculate the dew point is not defined for relative
humidity of 0.0 %. It is every unlikely to ever happen in real world
weather, but is handled just in case by using the smallest possible
numerical double value.

Reviewers: #swift_pilot_client

Maniphest Tasks: T406

Differential Revision: https://dev.swift-project.org/D82
This commit is contained in:
Roland Rossgotterer
2018-12-26 02:32:38 +01:00
committed by Mat Sutcliffe
parent 3db811f11d
commit 90bc693509
1 changed files with 13 additions and 3 deletions
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16
src/plugins/weatherdata/gfs/weatherdatagfs.cpp
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@@ -729,10 +729,20 @@ namespace BlackWxPlugin
CTemperature CWeatherDataGfs::calculateDewPoint(const CTemperature &temperature, double relativeHumidity)
{
// https://en.wikipedia.org/wiki/Dew_point#Calculating_the_dew_point
const double a = 6.112;
const double b = 17.67;
const double c = 243.5;
double temperatureInCelsius = temperature.value(CTemperatureUnit::C());
double saturationVaporPressure = 6.112 * std::exp((17.67 * temperatureInCelsius) / (temperatureInCelsius + 243.5));
double vaporPressure = saturationVaporPressure * (relativeHumidity / 100.0);
double dewPointInCelsius = std::log(vaporPressure / 6.112) * 243.5 / (17.67 - std::log(vaporPressure / 6.112));
double saturationVaporPressure = a * std::exp((b * temperatureInCelsius) / (c + temperatureInCelsius));
double vaporPressure = (relativeHumidity / 100.0) * saturationVaporPressure;
// std::log(0.0) is not defined. Hence we use the smallest value possible close to 0.0, if RH is 0.0.
// In real weather, RH 0.0 % will never exist.
vaporPressure = qMax(vaporPressure, std::numeric_limits<double>::min());
double dewPointInCelsius = c * std::log(vaporPressure / a) / (b - std::log(vaporPressure / a));
return { dewPointInCelsius, CTemperatureUnit::C() };
}